Current monitoring power regulator for microwave oven



Oct. 21, 1969 L. 5. SMITH 3,474,343

CURRENT MONITORING POWER REGULATOR FOR MICROWAVE] OVEN Filed Dec. 12, 1966 IIIIIIIHIIIIIIIH HIIIIIIIHHIIIIH //V 1 6 7 0L Zeanar/ J: Ina/ Af/UME) United States Patent US. Cl. 328-8 10 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a power regulator for a microwave oven, and more particularly, to an improved regulator which prevents the plate current of a magnetron from exceeding a maximum level, automatically, without the use of resettable fuses.

In microwave ovens, microwave energy is utilized for heating foods contained within a cavity. The source of the microwave energy is usually a magnetron, connected for oscillation at a predetermined microwave frequency, supplied with plate or anode current and voltages from a power source. Such power source is generally the ordinary 220 volt 60 cycle AC line voltage applied to the primary winding of a transformer and stepped up in the secondary winding of the transformer to the high voltage necessary to operate the magnetron. The high AC voltage may be applied directly to the magnetron or may be first applied to a bridge rectifier where it is rectified into a pulsating DC voltage and then applied to the magnetron. In the former instance, where high AC voltage from the secondary winding of the transformer is directly applied to the magnetron, the plate current, which flows, appears as a pulsating or half-wave rectified DC current due to the rectifying or diode action of the magnetron itself.

With either type of power source, the magnetron must be protected against an overload in the plate current. An overload results in excessive heating and destruction of the megnetron elements. The maximum value of plate current, which a particular magnetron can dissipate without destruction, is initially determined by design of the tube elements. Excursions of current above this maximum level destroy the tube. Too great a microwave load upon the tube output and an increase in the ordinary line voltage used as the power supply for the magnetron are two factors which cause an overload. Some means, therefore, must be utilized to prevent these overloads and to avoid the obvious magnetron replacement problem otherwise encountered.

To avoid obvious maintenance expenses, the form of an overload protection or regulation circuit should not require the attention of an operator and should contain only a few simple parts which are ordinarily reliable, uncompuicated, and inexpensive. However, even though a fuse is simple, it is best if manual replacement or manual resetting required of a common fuse can be avoided. Moreover, other automatic types of protectors prevent use of the oven until the condition has corrected itself and are less desirable because they contain corrodible contacts. It is also best if the relatively complex and expensive types of power regulators, which have heretofore been available for preventing the overload of the magnetron, and which require tedious adjustments by skilled personnel, can also be avoided.

Therefore, it is an object of the invention to provide a current overload protection circuit which automatically prevents the plate current of a magnetron from exceeding a predetermined level and which does not require manual resetting.

It is a further object of this invention to provide a current regulating circuit, which consists of relatively few simple, reliable, and inexpensive elements without corrodible contacts and automatically reduces the voltage applied to the magentron in order to prevent excessive plate current when the current attempts to exceed a predetermined maximum value without, by its operation, preventing operation of the magnetron.

It is a further object of this invention to provide a simple current regulating circuit for a magnetron utilized in a microwave oven which monitors the plate current of the magnetron and automatically reduces the Voltage applied to the magnetron when its plate current exceeds a predetermined maximum level so as to reduce the plate current and prevent the destruction of the magnetron.

In accordance with the invention, the current regulating circuit includes; a saturable reactor which has its secondary winding in circuit with a regulatable power source and which contains two primary windings, one connected in a first current conducting path, and the other one connected in a second current conducting path. The first current conducting path is connected in parallel with the second current conducting path between input and output terminals within a current monitoring circuit and contains a resistance in series between its associated primary winding and the input terminal. The second current conducting path contains a two-conductive-state voltage sensing device, such as a Zener diode, in series between its associated primary winding and the input terminal. The primary windings are arranged so that either winding individually may produce sufiicient magnetic flux to fully saturate the core of the saturable reactor with a given level of DC current therethrough, but are poled in opposite directions so that one winding produces magnetic flux oppositely poled to that produced by the other to reduce the net flux in the reactor core. In accordance with the invention, the aforesaid resistance contained in the first current conducting path is advantageously a nonlinear resistor having a positive resistance coeflicient, such as a tungsten filament lamp. Further, in accordance with the invention, the magnetron plate current is applied between the input and output terminals of the current monitoring circuit and passes only through the first current conducting path in order to fully saturate the core of the reactor wherein the secondary winding of the reactor normally appears to have a low value of inductance or impedance, while the second current conducting path does not conduct current since the two-conductive-state voltage sensing device in series therein, is normally in its nonductive state. Any increase in the monitored current over a predetermined maximum level causes an increased voltage to appear across the first current conducting path and results in current conduction through the second current conducting path enabled by the change of state of its voltage sensing device. Substantially, all of any incremental increase in monitored current above the maximum level will be diverted through the second current path, while the first path continues to conduct the maximum level of current. The incremental current, through the second primary winding, included in the second current path, produces a proportional amount of magnetic flux opposed to that produced in the first primary winding, resulting in a proportional amount of desaturation of the reactor core, and hence, makes the secondary winding of the reactor appear to have a larger inductance r impedance. In accordance with the invention, this change of impedance or inductance is utilized to regulate a power source by the circuit connection with the primary of a transformer supplying power to the magnetron.

The foregoing and other advantages and objects of the present invention will become apparent from a consideration of the following detailed description and drawings of which:

FIGURE 1 is a schematic diagram of an embodiment of the invention showing the current regulator in a microwave oven unit utilizing a power source in which high AC voltages are directly applied to the magnetron and in which the current regulator contains a circuit placed in series with the secondary winding of the transformer for directly monitoring the anode or plate current of the magnetron.

FIGURE 2 is a schematic of another embodiment of the invention in which the current regulator is in a microwave oven utilizing a power source containing a bridge rectifier to feed high voltage pulsating DC voltages to the magnetron.

FIGURE 1 shows a source of 220 volt 60 cycle AC connected by one line to a terminal 1 of the primary winding 2 of a voltage step-up transformer 3. The voltage step-up transformer 3 contains a secondary winding 4, whose terminal 5 is connected to the cathode 6 of a magnetron 7, the anode or plate 8 of which is connected to ground. A saturable reactor 9 contains a secondary winding 10, having one terminal 11 which is connected to terminal 12 of primary winding 2, and a second terminal 13 which is connected to the other line of the source to complete a series circuit to the 220 volt source. The saturable reactor 9 contains two primary windings, 14 and 15. The first primary winding 14 is connected in series within a first current conducting path between an input terminal 16 and an output terminal 17, through a center tap 18 to the primary winding connected to output terminal 17, and through terminal 24 to a resistance or a nonlinear resistor having a positive coefilcient of resistance and herein shown as a 12 volt tungsten filament lamp 19 to input terminal 16. The second primary winding 15 is also connected by means of the center tap 18 to the output terminal 17, and this winding is included in series within a second current conducting path between the input terminal 16 and output terminal 17 through a two-conductive-state voltage sensing device, herein shown as a Zener diode 20 connected between input terminal 16 and an input terminal 21 of the second primary winding 15. The first and second current conducting paths are, thus, in parallel between input terminal 16 and output terminal 17 and form therewith a current monitoring circuit.

A source of DC current, to be monitored and regulated, is applied between the input and output terminals 16 and 17. In the embodiment of FIGURE 1, this source is the magnetron plate current I. The current monitoring circuit is placed in series with the magnetron plate current; input terminal 16 is connected to terminal 22 of the secondary winding 4 of the transformer 3 and the output terminal 17 is connected to ground.

A capacitor 23 is connected between the input and output terminals 16 and 17 in order to smooth the DC voltage produced by the pulsating DC current, which flows through the resistance, tungsten lamp 19, as will hereinafter be discussed. Moreover, the primary windings of the saturable reactor 9 are poled so that the fiow of DC current into the winding 14 through terminal 24 produces a flux in opposition to the flux produced by DC current flowing into winding 15 through terminal 21. Zener diode 20 is poled so that it is back biased by the voltage drop which appears across resistance 19 and is normally nonconducting.

The saturable reactor 9 is a well known variable inductance or impedance device containing windings electromagnetically linked to an iron core in which the value of inductance or impedance presented by the secondary winding to an alternating current source placed thereacross depends upon the degree of DC magnetization of the core. The magnetization of the core may be varied from no magnetization to full saturation by a change in the level of DC current applied to a biasing or primary win-ding or windings, each of which produces magnetic flux in the core in response to the current flow therein. When the core is fully saturated, the secondary winding appears laS a very small impedance to AC voltages applied thereacross; however, when the core is not magnetized, the secondary appears as a high impedance. Between the magnetization levels of no magnetization and full magnetic saturation of the core, the effective value of impedance of the secondary varies as a function of the net magnetization of the core.

The voltage sensing device utilized in the invention in the second current conducting path is a well known Zener diode. A Zener diode, like an ordinary semiconductor diode, conducts current when voltage is applied in the forward direction, and does not conduct when voltage is applied in the backward direction. However, once the level of the voltage applied in the backward direction exceeds a given level or Zener breakdown voltage, the diode will breakdown and become conductive and current flows in the backward direction. The Zener diode, when conductive, maintains a relatively constant voltage drop thereacross at the breakdown level or Zener breakdown voltage, regardless of the level of current. In contrast to an ordinary diode, the Zener diode is designed to operate with current flowing in the backward direction without being destroyed and numerous Zener diodes are available with different desired Zener break down voltage levels. This characteristic of the Zener diode is utilized as a voltage responsive switch; the diode changes from its nonconductive state to its conductive state in response to a change in level of voltage applied in the backward direction from below to above the Zener breakdown voltage for the diode.

Upon the connection to the power lines, extending from the oven unit to the 220 volt AC line source 1 by a connector or a switch, neither of which is illustrated, current flows through the primary winding 2 of trans former 3 and through the secondary winding 10 of the saturable reactor 9. Assuming that conditions are normal, as will later be explained, the core of saturable reactor 9 is magnetically saturated, and the secondary winding 10 appears as a very small impedance to the applied alternating current. Hence, a very small voltage drop occurs across winding 10 and substantially the full 220 volts of the line source is applied across terminals 1 and 12 of primary winding 2. Transformer 3, in this embodiment, steps up the line source voltage to a higher voltage within the operating range of the magnetron. This high voltage appears across the secondary winding 4 of transformer 3. This high AC voltage is applied directly to the cathode 6 of the magnetron 7 and through the input and output terminals 16 and 17 of the current monitoring circuit and ground, to the plate 8 of the magnetron. The magnetron, like other diodes, acts as a halfwave rectifier when an AC voltage is applied between the plate and cathode. Hence, the magnetron conducts current only on alternating half cycles of the applied AC voltage. As a result, the current flows in the direction indicated in the figure and consists of a pulsating DC current. This DC current passes from transformer terminal 22 into input terminal 16 of the current monitoring circuit through output terminal 17 of the current monitoring circuit to ground, and from ground to the plate 8 to the cathode 6 and around to the terminal 5.

The magnetron is connected for oscillation at a microwave frequency and microwave frequency energy is removed from the tube by an output coupling 25 and fed into a conductively walled cavity in which food is received for heating. Such connections and details of both the oven and the magnetron are conventional and well known and are omitted for clarity.

In the normal operation of FIGURE 1, the pulsating DC plate current of magnetron 7 is within its allowable range. The DC current I applied to the current monitoring circuit flows only within the first current conducting path defined by input terminal 16; the tungsten lamp 19 primary winding 14 of saturable reactor 9 and out the center tap 18 to the output terminal 17 to ground. This normal plate current applied to primary winding 14 is sufficient to fully saturate the core of the saturable reactor. Hence, the secondary winding normally appears to be a small impedance. Therefore, substantially all of the voltage from 220 volt line source is applied to the primary winding 2 of transformer 3. With current I flowing a voltage drop is developed across the tungsten fila ment lamp 19. In the embodiment of FIGURE 1, this voltage drop places input terminal 16 at a positive polarity relative to output terminal 17 which is at ground potential. This voltage drop is applied across Zener diode 20, since the second current conducting path containing the diode 20 is also connected between input terminal 16 and output terminal 17. Zener diode 20 is poled so that its cathode is connected to the input terminal 16 and it is thus back biased. Zener diode 20 is selected to have a breakdown voltage such that when the current 1 is within its normal range, the Zener diode is in its nonconductive state and does not conduct current, since the voltage drop appearing across the resistance 19, hence across terminals 16 land 17, is less than the Zener breakdown voltage. Therefore, current normally does not flow through the second current conducting path and flows only through the first current conducting path.

However, if the current I exceeds the predetermined current level, the voltage drop across resistance 19 reaches the Zener breakdown voltage level, causing the Zener diode 20 to break down and conduct current therethrough. Any attempt to increase the voltage across the input and output terminals 16 and 17 results in the conduction of further current through the Zener diode. Due to the characteristics of the Zener diode, no larger voltage than its breakdown voltage can be sustained across it, and thus an increase in current through the first current conducting path, and thus through resistance 19, which would logically result in such a larger voltage drop across the diode is precluded. Therefore, substantially all of any increment in plate current AI above the predetermined maximum is substantially entirely diverted through the second current conducting path and into primary winding of the saturable reactor 9.

The incremental level of DC current AI flowing through winding 15 produces a proportional amount of flux in a direction opposing that flux produced by the current I flowing through winding 14, and effectively desaturates the core of the saturable reactor. Proportionately, the effective value of impedance presented by the secondary winding 10 of the saturable reactor increases and results in a larger voltage drop thereacross. Because a larger portion of the 220 volt line source is applied across the secondary winding 10 of the saturable reactor 9, appearing as a voltage drop, the remaining portion appears at the primary 2 of transformer 3 at a reduced level than theretofore. Since the voltage at the primary of the transformer 3 is reduced, the high voltage appearing across the secondary winding 4 is likewies reduced from its former value. Accordingly, since the voltage applied across the anode 8 and cathode 6 of the magnetron 7 is reduced, the plate current I is likewise reduced; hence, an overload is automatically prevented. The second current conducting path continues to correct or decrease the plate current by the described reguator action until the plate current has been reduced to a normal level less than the maximum permissible current by removal of the cause of the overload, such as restoration of the line voltage to a proper lower level. Thus, the disclosed invention automatically protects the magnetron against the destruction caused by excessive plate currents.

A resistor, not illustrated, may be placed in series between the center tap 18 and the output terminal 17, in order to allow the level of plate current flow to be conveniently measured.

In the preferred form of this invention, the resistance 19 placed in the first current conducting path has advantageously been a simple and inexpensive 12 volt tungsten filament lamp. The tungsten lamp, as is well known, is a nonlinear resistance having a positive temperature coefiicient of resistance. Thus, when the current I through the current conducting path containing the lamp increases, the resistance of the tungsten filament lamp 19, also increases. While the increased resistance acts slightly to reduce the plate current flow in the first current conducting path, it is of greater significance that a larger voltage drop is obtained across the lamp 19, than would be available with an ordinary resistor having the same current flowing therethrough. Because this voltage drop builds up more rapidly with an increase in current than with an ordinary resistor, the breakdown voltage of the Zener diode is reached more quickly. Thus, overload protection is ensured and enhanced. Further, because of the realtively low values of voltage drop across the lamp 19, Zener diodes have been found to satisfy the desired properties of a voltage sensing means, although for larger voltages, various well known gaseous conducting tubes may be utilized.

Because the maximum allowable current, for which overload protection is desired is dependent upon the type of magnetron tube utilized, the current monitoring circuit is readily adjustable. Accordingly, with a given saturable reactor, the voltage drop across the first current conducting circuit may be adjusted by inserting other lamps in series or by inserting additional fixed linear resistors in series therewith to provide a desired range of voltage drops which are a function of the current passing through the circuit. Likewise, the current level at which triggering of the voltage sensing means or Zener diode 20 may occur, can be accomplished by selection between available Zener diodes having different breakdown voltages at which the Zener diode changes from its nonconducting to its conducting state.

FIGURE 2 contains the same regulator circuitry utilized with the magnetron of FIGURE 1 and like elements have been similarly labeled. FIGURE 2, however, shows a different means of supplying plate current to the magnetron that is also commonly utilized in present microwave ovens. The only difference between FIGURE 2 and FIGURE 1 is that the latter contains a bridge rectifier 26 comprising diodes 27, 28, 29, and 30. The input arms of the bridge are connected across the terminals 5 and 22 of the secondary winding 4 of transformer 4. In order to place the current monitoring circuit in series with the magnetron plate current, one of the output arms of the bridge rectifier is connected to input terminal 16 of the current monitoring circuit, while the plate 8 of magnetron 7 is connected to ground to complete one side of a series current path between the bridge rectifier, through the current monitoring circuit, grounded at output terminal 17 and the magnetron plate. And the other output arm :of the bridge rectifier is connected directly to the cathode 6 of the magnetron 7 to complete the current path. The bridge rectifier 26 produces a pulsating DC voltage of one polarity during each half of the alternating current input cycle. This results in a much smoother voltage and accordingly the smoothing capacitor 23 may be reduced in size or deleted if desired. Operation of the regulator elements in FIGURE 2 is the same as that described 7 with respect to FIGURE 1. Therefore, no further discussion thereof appears necessary and reference is made to the discussion presented with respect to FIGURE 1.

In both the embodiments of FIGURE 1 and FIGURE 2, terminal 17 has been shown grounded. However, without departing from the invention, it is apparent that this terminal maybe connected directly to the plate 8 in either FIGURE 1 or FIGURE 2, which connection may be entirely isolated from ground as in the situation with other types of tubes wherein the cathode is preferably grounded, since one of the purposes of a like connection to ground is merely to complete an electrical circuit therebetween. As is apparent, such a modification, does not detract from the operation of the circuit as described. Moreover, the magnetron plate is grounded for practical considerations with a negative voltage supply connected to the cathode. With other types of tubes that require or prefer the cathode 6 to be -grounded, terminal 17 is isolated from ground and connected directly to plate 8 without departing from the invention. In addition, the direction of plate current through the current monitoring circuit may be reversed so that current normally may flow into terminal 17 and out of terminal 16. In such a case, the position of the Zener diode is reversed so that its cathode is connected to terminal 21 instead of input termnal 16. For example, in FIGURE 1, terminal 22 can be connected to terminal 17 and plate 8 to terminal 1'6. With respect to FIGURE 2, should it be desired to connect the positive terminal of the bridge rectifier 26 to ground and supply the positive DC voltage to the plate 8 by connecting the plate directly to input terminal 16, the regulator circuit can be utilized by simply reversing the Zener diode 20 so that its cathode is connected to terminal 21, rather than to input terminal 16.

Additionally, FIGURE 2 may be modified in accordance with the invention by connecting the positive terminal of the rectifier directly to plate '8, grounding the negative terminal of the rectifier and connecting the cathode 6 of magnetron 7 directly to the input terminal 16.

The foregoing embodiments of this invention have been described for purposes of illustration only and are not intended to limit the invention as set forth within the spirit and scope of the appended claims.

What is claimed is:

1. A current monitoring regulator adapted for the power system of a microwave oven comprising:

a saturable reactor having a core of magnetic material,

a secondary winding for regulating a power source, a first primary winding for producing flux in said core, and a second primary winding for producing an oppositely poled flux in said core; and

a current monitoring circuit, said current monitoring circuit including an input, an output, a first current conducting path connected between said input and for conducting all current below and including a predetermined level applied between said input and output, wherein said first current conducting path includes, in series therein, a resistance means responsive to current flow therethrough for producing a voltage across said input and output, and a second current conducting path connected between said input and output, normally nonconductive and responsive to an increase of current above said predetermined level applied between said input and output for conducting only that portion of applied current in excess of said predetermined level, said second current conducting path includes, in series therein, a voltage sensing means having a first nonconductive state and a second conductive state and a substantially constant voltage characteristic in said second state responsive to a predetermined level of voltage across said input and output for switching from said first nonconductive state to said second conductive state and said first conducting path further including, in series therein, said first primary winding and said second current conducting path further including, in series therein, said second primary winding.

2. The invention as defined in claim 1, wherein said voltage sensing means comprises a Zener diode.

3. The invention as defined in claim 2, wherein said resistance means comprises a tungsten filament lamp.

4. The invention as defined in claim 2, further comprising a source of AC power; a transformer having a transformer primary winding and transformer secondary winding; means connecting said source, said transformer primary winding, and said secondary winding of said saturable reactor in series for permitting said reactor to regulate the output of said transformer; an electrical load; and electn'eai means connecting said electrical load to said transformer secondary winding and including, in series therewith, said input and output of said current monitoring circuit for supplying current to said load and to said current monitoring circuit for regulating the current supplied to said load.

5. The invention as defined in claim 4, wherein said load comprises a magnetron.

6. The invention as defined in claim 5, wherein said electrical means includes a bridge rectifier.

7. The invention as defined in claim 3, further comprising a source of AC power; a transformer having a transformer primary winding and transformer secondary winding; means connectin gsaid source, said transformer primary winding and said secondary winding of said saturable reactor in series for permitting said reactor to regulate the output of said transformer; an electrical load; and electrical means connectingsaid electrical load to said transformer secondary winding and including, in series therewith, said input and output of said current monitoring circuit for supplying current to said load and to said current monitoring circuit for regulating the current supplied said load.

8. The invention as defined in claim 7, wherein said load comprises a magnetron.

9. The invention as defined in claim 8, wherein said electrical means includes a bridge rectifier.

10. In a microwave oven including a magnetron for providing a source of microwave energy, the combination comprising, a source of alternating current; a transformer having a primary winding and a secondary winding for converting the voltage 0 fsaid alternating current source to a high voltage for operating said magnetron; a regulator circuit comprising a saturable reactor having a first and second primary winding and a secondary winding; means for connecting said secondary winding of said reactor in series between said alternating current source and said primary winding of said transformer for controlling the high voltage output of said secondary winding of said transformer; and a current monitoring circuit comprising an input, an output, a first current conducting path and a second current conducting path connected in parallel with said first current conducting path between said input and output, said first current conducting path including said first primary winding of said saturable reactor in series with a tungsten filament lamp responsive to current therethrough for producing a voltage between said input and output, said second current conducting path including said second primary winding of said saturable reactor in series with a Zener diode normally nonconductive and responsive to a predetermined level of voltage across said input and output for changing to its conductive state, and means for connecting said secondary winding of said transformer, said magnetron and said input and output in series to permit said current monitoring circuit to monitor the magnetron current, whereby the magnetron current flowing into said current monitoring circuit normally flows only through said first current conducting path to saturate the core of said saturable reactor and the voltage drop across the tungsten lamp is insufficient to cause the Zener diode to change to its conductive state but wherein any increase in current flowing in said first current conducting path above 9 a predetermined maximum creates a voltage drop sufiicient to cause said Zener diode to break down and conduct current through said second current conducting path causing said second primary winding to create a magnetic flux in the core proportional to such current and opposing the flux created by the first primary winding of 5 said reactor resulting in an apparent increase of impedance across the secondary Winding of said reactor which reduces the current source voltage appearing across said tecting the magnetron against overload.

10 References Cited UNITED STATES PATENTS 3,317,699 7/1967 Helfer 21910.55 3,365,675 1/1968 Gaddy et a1. 330-192 JOHN W. HUCKERT, Primary Examiner SIMON BRODER, Assistant Examiner US. Cl. X.R.

Patent No.

Inventor(s) l M'H'Ll) S'IA'I'ES PA'IENT OFFNJE Dated October 21, 1969 LEONARD S. SMITH It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

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Edward M. Fletcher, 11-

Attesting Officer "megnetron" should be --mo.gnetron-- "and an increase" should be -or an increase-- "termndl" should be --cerminc1l-- smum mo SEALED JUN 9 1970 WILLIAM E. 'S'OIHUYLER, .TR. Commissioner of Patents 

